FIG. 1 shows a combustion and flue gas treatment systems such as those systems commonly used in power plants; the system 1 has a furnace 2 (for example being part of a boiler) where a fuel is combusted with an oxidizer, for example oxygen or air enriched oxygen.
The flue gas produced in the furnace 2 is forwarded to a NOx removal unit 3, such as a SCR, where the flue gas is washed in counter current against water containing a reagent for NOx removal. The flue gas is then passed through a heat exchanger 4 where it is cooled, and is then forwarded to a dust removal unit 5, such as filters or electrostatic precipitators.
The cooled flue gas deprived from NOx and dust is then passed through a SOx removal unit 6, where the flue gas is washed again against a reagent for SOx removal (possibly contained in a slurry) at a flue gas temperature of about 120° C. The flue gas also deprived from SOx is thus forwarded to a direct contact cooler 7 such as a condenser, where water is condensed and removed from the flue gas. The pre-treated flue gas is thus forwarded to a gas processing unit or CPU 8 where carbon dioxide is separated from the other gas of the flue gas. Carbon dioxide is thus forwarded via a line 9 to storage or further treatments or use; the other gas is forwarded to the stack 10 for being vented to the atmosphere.
In addition, in order to increase the carbon dioxide content of the flue gas, a part of the flue gas is recirculated back into the furnace 2. In this respect, the system 1 has a recirculation line 11 that departs from downstream the SOx removal unit 6, for example upstream or downstream the direct contact cooler 7.
The recirculation line 11 is connected with the heat exchanger 4, such that the recirculated flue gas (actually partly clean flue gas) is heated and the flue gas directed into the dust removal unit 5 is cooled.
This traditional design has some disadvantages.
In fact, at the heat exchanger 4 oxygen slip (i.e. oxygen passage into the flue gas) can occur; this is disadvantageous because oxygen contaminates the flue gas and must be removed in order to obtain high purity carbon dioxide from the flue gas.
In addition, since the NOx removal unit 3 is immediately downstream of the furnace 2, deactivation of the catalyzer typically occurs.